Unable to publish UWP package update with the `invalid package identity/family name` error

Question

I'm trying to upload a UWP package on top of an existing published WP8.x Windows Phone app:

1. I've created a new UWP project
2. I hit the Associate app with the store menu and picked the already published app
3. After having my manifest updated by VS2017 automatically I've created a package with the Create app packages menu
4. Any attempt to upload the package giving me the following error:

MyPackageName_x86_x64_arm_bundle.appxupload 22.0 MB Invalid package identity name: IdentityNameFromManifest (expected: ) Invalid package family name: IdentityNameFromManifest_SomeHash (expected: )

Similar issues are reported by other developers but in all cases, it was the wrong store app association where one identity was set and another is expected. In my case, the expected identity is empty for some reason and I did my store association so brand new app created from scratch so it shouldn't be some legacy codebase issue.

1. What could be wrong with my pakcage identity/family name?
2. Why the expected signature reported by the store upload UI is empty?